Re: [PATCH 1/2] block: Add support for atomic writes

[Howard Chu <hyc@symas.com>]

Chris Mason wrote:
> Quoting Matthew Wilcox (2013-11-12 10:11:51)
>> I took a look at the SCSI Block Command spec.  If I understand it
>> correctly, SCSI would implement this with the WRITE USING TOKEN command.
>> I don't see why it couldn't implement this API, though it seems like
>> SCSI would prefer a separate setup step before the write comes in.  I'm
>> not sure that's a reasonable request to make of the application (nor
>> am I sure I understand SBC correctly).
>
> What kind of setup would we have to do?  We have all the IO in hand, so
> it can be organized in just about any way needed.
>
>>
>> I like the API, but I'm a little confused not to see a patch saying "Oh,
>> and here's how we implemented it in btrfs without any hardware support"
>> ;-)  It seems to me that the concept is just as good a match for an
>> advanced filesystem that supports snapshots as it is for the FTL inside
>> a drive.
>
> Grin, almost Btrfs already does this...COW means that btrfs needs to
> update metadata to point to new locations.  To avoid an ugly
> flush-all-the-io-every-commit mess, we track pending writes and update
> the meatadata when the write is fully on media.
>
> We're missing a firm line that makes sure all the metadata updates for a
> single write happen in the same transaction, but that part isn't hard.
>
> We're missing good performance in database workloads, which is a
> slightly bigger trick.

This is precisely why this is needed:
http://www.spinics.net/lists/linux-fsdevel/msg70047.html

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/